Provide loadBeforeEx

[navytux]

loadBeforeEx is like loadBefore, but simpler, provides better
information for object delete records and can be more efficiently
implemented by many storages: zopefoundation/ZODB#323

On RelStorage loadBefore is currently implemented via 3 SQL queries:

1. check whether object record exists at all
2. retrieve object state
3. retrieve serial of next object revision

Compared to that loadBeforeEx is implemented via only one SQL query "2"
from the above - "retrieve object state". It is exactly the same query
that loadBefore uses and after the patch loadBefore actually invokes
loadBeforeEx for step 2.

This change was outlined in

zopefoundation/ZODB#318 (comment) and
zopefoundation/ZODB#318 (comment)

and as explained in the first link this patch is also semantically coupled with

#484

This patch passes tests with both ZODB5 and with ZODB5+zopefoundation/ZODB#323:

• when ran with ZODB5 it verifies that loadBefore implementation does
  not become broken.

• when ran with ZODB5+loadAt zopefoundation/ZODB#323 it
  verifies that loadBeforeEx implementation is correct.

For tests to pass with
ZODB5+zopefoundation/ZODB#323 we also need
zopefoundation/zc.zlibstorage#11 because without
that fix zc.zlibstorage does not decompress data on loadBeforeEx.

[navytux]

Regarding test failure: I tend to believe that the sole failue of PYTHON=C:\Python38-x64 is due to some flakiness and is unrelated to hereby patch becuase of the following reasons:

• That relstorage.adapters.sqlite.tests.test_sqlite.Sqlite3RSDestHFZodbConvertTests_sqlite3 passes locally for me, and it passed in all other runs: PYTHON=C:\Python27-x64, PYTHON=C:\Python36-x64 and PYTHON=C:\Python37-x64.
• When run with regular ZODB5 (without loadAt zopefoundation/ZODB#323) hereby patch semantic is only slight code movement.

Kirill